1. Field of the Invention
The present invention relates to apparatus for measuring transparent aspheric surfaces. Examples of such surfaces include the cornea of the eye and the surface of a contact lens. More particularly, the invention combines a optical system with a video image analysis system to generate a mathematical expression describing the shape of a transparent aspheric surface.
2. Description of the Art
In the prior art it has been recognized that accurate characterization of the shape of the surface of the cornea would aid in the fitting of contact lenses. However, initially it was believed that the cornea had a substantially spheric shape and that the xe2x80x9cfitxe2x80x9d between the cornea and the lens need not be exact, for the comfort of the wearer or for operation of the lens. In general, it was believed that xe2x80x9csoftxe2x80x9d contact lens material would be compliant enough to conform to the corneal shape, and that only a few xe2x80x9cbase curvesxe2x80x9d would suffice to fit the majority of the population. Lindmark et al. demonstrated that the corneal shape is not spherical and that failure to accommodate the complex surface of the cornea can result in injury to the cornea itself, see xe2x80x9cThe Correction of Atypical Ametropia with Flexlensxe2x80x9d, Vol. 13, 1979, of the Contact Lens Journal, R. C. Lindmark, et al.
In the prior art, there have been two principal techniques used for estimating the xe2x80x9csphericityxe2x80x9d of the cornea. The earliest systems involve the projection of Placido""s rings onto a cornea. The practitioner observes the clarity and spacing of the projected rings and compares the resultant image pattern with reference curves to estimate which one of a collection of spherical curves most closely approximates the surface of the cornea. The earliest systems of this type relied on direct observation of the projected rings on the eye. More recent versions of this system photograph the ring pattern on the eye producing a karotograph which may be evaluated with the use of a computer system. Examples of this approach are taught by U.S. Pat. No. 4,685,140 to Mount; U.S. Pat. No. 4,978,213 to El Hage; In each of these systems a television camera is utilized along with a computer to process the Placido""s ring data.
An alternate approach develops an image of the cross section image of the meridian of cornea of the eye with an optical system. This approach is typified by the Corneoptor system developed by Scientific Advances Incorporated of Columbus, Ohio in the late 1960""s. This system utilizes a slit illuminator to develop a photograph of the cross section of the cornea. In use the operator would compare the test curves to determine the xe2x80x9cbest fitxe2x80x9d representation of the contour.
It is now well recognized that the cornea of the eye has a very complex aspheric shape, and that more exact knowledge of this shape for particular individuals would be an aid to fitting contact lenses. Such a system would also be useful for evaluating the corneal surface for surgical procedures and for evaluating the base curve and front curve of contact lenses as an aid to fitting them on the human eye.
In one aspect, the present invention provides a flexible measurement tool for assessing the shape of complex aspheric transparent surfaces. For purposes of this disclosure the term xe2x80x9csubject surfacexe2x80x9d may include any transparent surface, of which the cornea and contact lens and a contact lens on a cornea are only examples.
The apparatus of the invention includes four subsystems.
The subject surface itself is held by a positioning subsystem which preferably is a wet cell. A measurement reticle located near the subject surface may be presented along with the subject surface as an aid to calibration of the system. However it is preferred to substitute a lens with known curved surfaces into the preferred wet cell to calibrate the system.
The illumination subsystem projects a slit of light onto the subject surface along an illumination axis. An image formation subsystem is aligned along an observation axis which is orthogonal to the illumination axis. Together these two subsystems present a cross-section image of the subject surface to a camera which is associated with the image formation subsystem. The illumination subsystem and image formation subsystem can be moved with respect to the subject surface lens so that cross-sectional data sets can be taken at several positions. The camera generates a cross-sectional image of the subject surface referred to as the xe2x80x9craw dataxe2x80x9d This signal digitized to form a xe2x80x9cpixel data setxe2x80x9d. The conversion process may occur within the data processing subsystem which includes a computer. Within the data processing subsystem the pixel data set converted to a silhouette image data set. This is accomplished by an image contrast enhancement process which applies a threshold criteria to the pixel data set and generates a white-on-black silhouette of the particular crossection selected by the orientation of the illumination subsystem. Next Cartesian coordinates are applied to the silhouette image data set. The coordinates are applied to the silhouette image to define a first surface data set and a second surface data set. This process may involve editing the data sets to exclude non-lens areas of the subject surface. The first and second surface data sets are compared with regular conic section equations to ascertain whether they adequately represent the data sets. Assuming the conics are not suitable, the data processor computes a higher order polynomial expression for the first and second surface data sets which is displayed to the user along with an image of subject surface. Preferably a least means squares algorithm is applied to the data set to generate a polynomial expression for the curve displayed by the subject surface although a finite element analysis may be used as well.